[chrisco255]

The government can rule it however they want. They can use existing trust-busting precedent to pick apart the large firms if they want to, just like Standard Oil, AT&T, etc. It would take an act of Congress or the FTC. It would take a lot, but it could be done. Microsoft fit your definition with 90+% of desktop OS market share in the 90s and they had to settle with the government (https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor...). That the government has been asleep at the wheel as Apple, Amazon, Facebook and Google have risen up is irrelevant. All it would take is an aggressive anti-trust leaning Congress or President to shake things up again.